We are investigating and further characterizing the RNA metabolism of cells infected with RNA tumor viruses. Most of the experiments involve mouse cells infected with murine leukemia virus (MuLV), and in some cases other cells and other RNA tumor viruses will be used. Virus-specific RNA in these experiments is detected by RNA-DNA hybridization of cellular RNA with virus-specific DNA synthesized by the "endogenous" reverse transcriptase reaction in MuLV. Three topics are being studied: (1) Virus-specific RNAs from non-producer mouse cells and cells with partial virus expression are being identified and characterized by annealing cellular RNA with H3 DNA probe prepared from endogenous C-type viruses. (2) Labeling kinetics of virus-specific RNA in MuLV infected cells are being studied and characterization of rapidly-labeled virus-specific RNAs is being performed. These RNAs are identified by hybridization to excess virus-specific DNA attached to paper filters. (3) We are attempting to develop immune precipitation of virus-specific polyribosomes from infected cells. Using monospecific antisera for different virus proteins, we hope to separate and identify different virus-specific messenger RNAs. BIBLIOGRAPHIC REFERENCES: Fan, H., and P. Besmer. RNA Metabolism of Murine Leukemia Virus. II. Endogenous Virus-Specific RNA in the Uninfected BALB/c Cell Line JLS-V9. J. Virol., 15, 836-842 (1975). Jaenisch, R., H. Fan, and B. Croker. Infection of Preimplantation Mouse Embryos and of Newborn Mice with Leukemia Virus: Tissue Distribution of Viral DNA and RNA and Leukemogenesis in the Adult Animal. Proc. Nat. Acad. Sci. USA, 72, in press (1975).